2000 S80 Low Compression - potential DIY Ring Replacement

[boosted5cyl]

IMO its probably not worth doing rings AND top end with the GM transmission and two turbos in there. Its a matter of personal preference I suppose. I would fix the misfire and see if performance and fuel economy are acceptable. 4 has low compression but might be excessive wear from being fuel diliuted///

Anyway
Given that the cat is glowing on bank#2, it must be getting fuel from the injectors.... I suspect the plugs on 4 & 5 are not firing. That could be a plug issue, coil issue, coil wiring or a combo of any three. Some ideas to try...

Swap a coil from either 4 or 5 with a know working cylinder and see if the misfire moves, ie do the test you did already by disconnecting coils\injectors.
or
Disconnect the injectors on 4 & 5, pull the plugs and coils, ground them and see if they spark. Maybe throw new plugs in those two just to be safe. If they spark, its not a 100% guarantee they are firing under compression but at least you know they are doing "something". You could then swap coils with a know good one, say #6. If a spark appears then the coil is a dud, if not then you need to look at wiring. If my '99 is anything to go by the wiring in these gets pretty brittle and nasty under there.

Provided the cat isnt clogged I think you'll find the performance to be acceptable once to cure the misfire, even with those low compression numbers.
You could try an autoRX treatment in the oil, or even a more agressive engine flush to decoke the ring packs.

[Oly_850]

Update on my post from several weeks ago.

Removed head that had already been surfaced and took it to a different machine shop. Diagnosis was that the valves on 4 & 5 definitely needed work, but that all the exhaust valves (and guides) were leaking as well. Had an full valve grind done to the tune of $420.

Reassembled over 2.5 days and ran it this afternoon. Huge improvement. Got it warm and did a compression test: 160, 166, 155, 152, 151, 169. No more glowing catalytic converter, runs on 6 cylinders, has power, etc.

Now working the following:

Once the car goes into closed loop operation, a 'loping' idle, but drives fine with the obvious exception of the transmission problem to sort out. Have the following codes, indications:

P0756 - Transmission solenoid, I know, I know...
Transmission Service Urgent notification
P0032 (pending) - High Voltage O2 sensor bank 1 sensor 1
P0141 - O2 Sensor heater circuit malfunction bank 1 sensor 2
P1618 - Torque converter switch / valve body (?)

Also, need to service the flame trap, have positive crankcase pressure and vapor at the dipstick.